Sealing is oftentimes essential when there are two relatively movable mechanical members in close proximity to one another. The members may have substantial relative motion, such as a turbine shaft against a lubricating oil reservoir, or a rotatable turbine stage relative to fixed support structure and having to withstand a pressure differential across the stage. Alternatively, there could be small vibratory pulsing motion between the members, along with thermal growth, as in sealing between a combustor and a transition piece. The resultant dynamic mismatch at typical combustor junctions requires an effective seal to contain the combustion products and a pressure differential and to allow for the dimensional growth.
More particularly, a plurality of combustors are conventionally disposed in an annular array about the axis of a turbine. Hot gases of combustion flow from each combustor through a transition piece into the first-stage nozzle. Because the transition pieces and the first-stage nozzle are formed of different materials and are subjected to different temperatures during operation, they experience different degrees of thermal growth. Thus, both the transition pieces and the first-stage nozzle support elements move radially, circumferentially and axially relative to one another as a result of thermal growth. Dynamic pulsing between these parts also occurs. This time variable mismatch at the junction of the transition pieces and the first-stage nozzle support elements requires an effective seal to contain the combustion products and the pressure differential across the space, while accommodating these dimensional changes. More particularly, the sealing devices have to conform to surfaces that have time varying characteristics, including whirl, wobble and surface undulations introduced due to wear, and must be capable of operating under high temperature conditions.
A substantial number of seals have been employed in turbine applications in the past. For example, labyrinth seals are often employed between relatively rotating parts. Labyrinth seals, however, have limitations in that they do not easily conform to vibratory movement or rotating surfaces, particularly if there are imperfections in the surfaces. Labyrinth seals have not demonstrated usefulness in non-rotary applications. Brush seals have been used in many environments, including turbines. Brush seals generally conform better to surface non-uniformities and seal better than labyrinth seals. However, they are severely limited by handling and fabrication difficulties. For example, the very fine bristle wires are not bound together before assembly and it is a formidable task to lay out a predetermined layer of bristles to the required thickness to form a bristle pack suitable to form the resulting seal. The brush seals are, thus, very expensive.